1. Field of the Invention
The invention relates to recording and playback of signals from a magnetic tape. In particular, the invention relates to a bidirectional signal-transmission channel for optically coupling record and playback signals to and from a rotary magnetic head scanner.
2. Description Relative to the Prior Art
A digital recording system must be capable of handling a very high data rate if a large quantity of data is to be recorded in a relatively brief time interval. The need to enhance the data recording rate and packing density capability of a digital tape recorder of the rotary head scanner type places severe demands on existing methods for transferring a data signal to and from each magnetic head on the scanner.
Currently, the commonly used rotary transformer limits a data signal to a rotary head to about 50 megahertz (MHz). A currently proposed advanced digital tape recording system is to handle data at a 300 megabit per second (Mb/sec) rate. The minimum bandwidth for such data, however, is 450 MHz, which, of course, is well beyond the frequency range of rotary transformers known in the art. Even with the introduction of electronics integral to a headwheel and improved transformer characteristics, a rotary transformer has a predicted upper bandwidth of only approximately 150 MHz. Accordingly, a rotary transformer suffers from a disadvantage in its ability to handle the high data rate required for a future digital recording system.
It is known in the prior art that optical coupling of signals to and from a rotary head scanner offers potential advantages over a rotary transformer. Optical coupling permits broadband signal recording and reproduction with high efficiency and good signal-to-noise; also crosstalk between signals can be virtually eliminated.
European patent application No. 0 074 796 discloses apparatus for providing optical coupling to a rotary head scanner. For that purpose, a laser diode, located on the rotational axis of the headwheel, is intended to serve a dual function--light-emitting in a playback mode and light-receiving in a record mode. To those ends, an external mode switch functions to reverse bias the diode during a recording operation and to forwardly bias the diode during playback.
As evidenced by the need for an external mode switch, operating a laser diode in a dual mode adds complexity to the headwheel electronics. Furthermore, it is believed that, with a "dual-function" laser diode, the signal-to-noise ratio and bandwidth of the record and/or playback signals may be compromised as it would not be reasonable to expect optimum diode performance in both operating modes.
Copending U.S. patent application Ser. No. 819,668, filed Jan. 17, 1986, now abandoned, discloses a single optically coupled channel having separate optical paths for the record and playback signals. With two magnetic heads--one for recording and the other for playback--record and playback signals may be coupled contemporaneously to and from the head scanner, for testing and certifying the operation of the recording apparatus in real time.
To couple a record signal to the head scanner, a laser, located off-axis on the stationary side of the scanner, serves to direct a modulated beam of collimated light obliquely onto a first photodetector, mounted on the rotational axis of the headwheel. Similarly, to optically couple a playback signal, a separate laser, located on the headwheel slightly off-axis, projects modulated collimated light obliquely onto a second photodetector, also aligned with the headwheel axis on the stationary side of the scanner. As the headwheel rotates, light from the "playback" laser defines a conical surface of revolution, with the apex of the cone coincident with the "playback" photodetector.
To maximize the signal-to-noise ratio of an optical channel, it is necessary to optimize the electro-optical "action" at each photodetector. The cross section of a collimated light beam, measured at an imaginary reference plane of orthogonal to the beam, is, of course, different than the cross section of the beam, measured at an oblique plane. Accordingly, when the laser beam, at its point of origin, is circular and the light-receiving area of the photodetector is circular, as is usually the case, the size and shape of the cross section of the light beam, at its plane of incidence, would not be matched to the photoconductive surface. Thus, the optical channel would lose signal-to-noise ratio because the electro-optical transducing action would not be maximized.
When the light source is near the rotational axis, as taught in the aforementioned Ser. No. 819,668, dispersion of the light beam, due to its angle of approach, would be negligible. Although the optical channel of 819,668 has been found to work well for its intended purpose, it is not always possible to provide each light source in close proximity to the on-axis position of the photodetector, particularly with a modern head scanner of compact design.